Bring LDAP-related nav items together

Removes a duplicate entry, and brings related topics together.

I chose to situate everything under "auth" rather than "install" where some things were originally:

• Before: https://docs.gitlab.com/ee/administration/auth/ldap/
• After: http://eread-bring-ldap-nav-items-together.178.62.207.141.nip.io/ee/administration/auth/ldap/